1. Field of the Invention
The present invention relates to a developing unit using a two-component developer and an image forming apparatus comprising the same.
2. Description of the Related Art
As the developing unit to be applied to image forming apparatus such as electrophotographic printer and copying machine there is widely used a developing unit using a two-component developer composed of a toner and a magnetic powder called carrier. When such a two-component developer is stirred in a developer receiving portion, the toner and the carrier in the developer are rubbed with each other so that they are each charged to a predetermined degree. The developer which has thus been charged to a predetermined degree is introduced from the developer receiving portion into a developing roll comprising a development sleeve and a plurality of magnetic poles fixed in the interior of the development sleeve. The developer which has been supplied onto the surface of the developing roll is then conveyed as the developing roll rotates while being retained in the form of magnetic brush. The developer passes by a regulation member called doctor blade disposed in the vicinity of the periphery of the developing roll. Thereafter, the developer is conveyed to a development region opposed to a photoreceptor which is an image carrier.
Various configurations of developing unit using a two-component developer have been proposed. In particular, a high speed process which operates at a rate of 300 mm/s or more is subject to shortage of developability of electrostatic latent image and thus often employs a hybrid process developing unit as a method of raising developability. The hybrid process developing unit comprises a plurality of developing rolls having different rotary directions disposed opposed to the development region. In order to explain the hybrid process developing unit, the terms “forward rotation developing roll” and “reverse rotation developing roll” will be used hereinafter for convenience. The term “forward rotation developing roll” as used herein is meant to indicate a developing roll which rotates counterclockwise if the rotary direction of the photoreceptor is considered clockwise. In other words, as viewed on the development region, both the photoreceptor and the forward rotation developing move in the same direction. On the contrary, the term “reverse rotation developing roll” as used herein is meant to indicate a developing unit which rotates clockwise if the rotary direction of the photoreceptor is considered clockwise. In other words, as viewed on the development region, both the photoreceptor and the reverse rotation developing roll move in the opposite directions.
Among configurations comprising a forward rotation developing roll and a reverse rotation developing roll in combination, a developing unit comprising a reverse rotation developing roll disposed upstream in the direction of rotation of the photoreceptor, a forward rotation developing roll disposed adjacent to the reverse rotation developing roll downstream in the direction of rotation of the photoreceptor, and a double-edged doctor blade disposed interposed between the reverse rotation developing roll and the forward rotation developing roll is particularly called fountain type developing unit. As mentioned above, the fountain type developing unit is advantageous in that it exhibits high developing properties. The fountain type developing unit is also advantageous in that it is little subject to the occurrence of defects such as image tail loss and head loss caused by the rotary direction of developing rolls. The fountain type developing unit is further advantageous in that it can work with only one double-edged doctor blade, making it possible to render itself compact.
A doctor blade is provided for the purpose of trimming the top of the magnetic brush formed by the developer retained on the surface of the developing roll to keep the amount of the developer to be conveyed to the development region at a proper value. The developer which has been regulated to a predetermined amount by the doctor blade is conveyed by the rotation of the developing roll into the development region where the developer on the developing roller is then brought into contact with the photoreceptor to develop the electrostatic latent image. During this procedure, a bias voltage is applied to the developing roller for introducing and supplying only the toner into the image forming area among the non-image area and the image forming area constituting the electrostatic latent image. In this manner, a toner image is formed on the image forming area of the photoreceptor.
It has heretofore been usual that the doctor blade is disposed interposed between the magnetic poles of the developing rolls in the fountain type developing unit. This reason for this arrangement is to raise the conveyability of the developer on the development roll. Another reason is attributed to the presumption that when the top of magnetic brush is trimmed at the position having a small magnetic force, the developer can be less stressed. However, with the recent demand for higher image quality, it has been desired to realize uniform development with a thin layer. Thus, there has been growing demand for the reduction and uniformalization of the regulated amount of the developer. To this end, the gap between the doctor blade and the developing roll (hereinafter referred to as “doctor gap”) has been reduced more and designed more precisely.
The magnetic brush formed on the developing roll is arranged along the magnetic line of force. Therefore, in the region where the magnetic poles are opposed to each other, the magnetic brush is erected in the direction normal to the surface of the developing roll and thus is sparsely populated. In the region between magnetic poles, the magnetic brush lies down along the periphery of the developing roll and thus is densely populated. Accordingly, when the top of the magnetic brush is trimmed in the region between magnetic poles, the densely populated developer must be regulated. Thus, the change of the doctor gap has a great effect on the conveyed amount of the developer. Therefore, the doctor gap must be adjusted with a high precision.
In the case where the doctor blade is disposed opposed to the magnetic poles, on the other hand, the top of the magnetic brush is trimmed while it is erected in the direction normal to the surface of the developing roll and sparsely populated. Accordingly, even when the doctor gap is predetermined to be relatively great, the conveyed amount of the developer can be reduced. A method involving the predetermination of the doctor gap to a great value has been proposed as a method which allows the reduction of the effect of error in the adjustment of gap on the conveyed amount of the developer and hence stable realization of uniform development with a thin layer (e.g., JP-A-53-77530 and JP-A-63-24268). The development process using a doctor blade disposed opposed to the magnetic poles is referred to as “on-the-pole trimming development”.
However, the on-the-pole trimming development process is disadvantageous in that the conveyed amount of the developer must be regulated with the magnetic brush sparsely erected in the direction normal to the surface of the developing roll, making it difficult to secure the uniformity in the developer kept erected. It has been heretofore devised to raise the width of the distribution of magnetic poles in the doctor blade zone. In particular, a high speed printing process (process rate: 300 mm/s or more) requires that the developer be conveyed at a high speed. Thus, both the high speed conveyance of the developer and the uniform conveyance of the developer cannot be attained at the same time. This causes deterioration of image quality such as unevenness in print density. Further, when a developer having a fluidity reduced by the reduction of the particle diameter of carrier is used, a problem of clogging of doctor gap with developer can easily occur.